Method for the manufacture of molds using casting sand or another mixture of raw material particles

ABSTRACT

In order to make casting sand or another moldable mixture, a pressure surge wave is used which has upper and lower limit values which values have been established by an optimal selection of a pattern of pressures. The other moldable mixture comprises particles of raw material, binder, water, and, if need be, additives. The rise in pressure takes place with an increasing pressure gradient, dp/dt, of at least 50 atmospheres (absolute)/second. A minimum pressure of at least 2 atmospheres (absolute) is maintained for at least 0.01 second. The pressure drop takes place at a decreasing pressure gradient,--dp/dt, of up to about 2.0 atmospheres (absolute)/second. The disclosed values represent an optimalization, in view of a simple design and economy of operation, with a surprisingly good compacting.

FIELD OF THE INVENTION

The present invention relates to a method of manufacturing molds fromcasting sand or from an otherwise moldable mixture by the use of apressure surge wave in a closed chamber. The other moldable mixturecomprises particles of raw material, binder, water, and, if need be,additives.

BACKGROUND OF THE INVENTION

A number of processes are known whereby the manufacturing of molds iseffected by means of a pressure surge wave. In such processes, a gaspressure is built up over a moldable mixture such as, for example, acasting sand, and then subsequently decreased.

The object of the invention is to select a range in pressure variationwithin which an optimal compaction of the moldable mixture is assured.

SUMMARY OF THE INVENTION

In accordance with the present invention, castings are made from castingsand or other moldable mixture by means of a pressure surge wave. Thepressure surge wave is characterized by four parameters: these are theincreasing pressure gradient, the maximum pressure, the duration of themaximum pressure, and the decreasing pressure gradient. The rise inpressure takes place with an increasing pressure gradient, dp/dt, of atleast about 50 atmospheres (absolute)/second. A .[.maximum.]. pressureof at least about 2 atmospheres (absolute) is maintained for at least0.01 seconds. The pressure drop takes place with a decreasing pressuregradient, -dp/dt, of up to 2.0 atmospheres (absolute)/sec. These valuesyield surprisingly good compacting of the casting sand and or othermoldable mixture from which the castings are made.

BRIEF DESCRIPTION OF THE DRAWINGS

The FIGURE shows four pressure surge waves as a function of time, inaccordance with an illustrative embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The pressure range is characterized bythe fact that the moldable mixtureis exposed to a gas pressure of at least 2 atmospheres (absolute) usingan increasing pressure gradient, dp/dt, of at least 50 atmospheres(absolute)/second, maintaining at least this pressure for at least 0.01seconds and then allowing the pressure to drop back to normal pressurewithin at least 0.2 seconds from the onset of the pressure increase.

Theoretically, no upper limit is set for the pressure-rise gradient.

Several tests have shown that an increasing pressure gradient of about600 atmospheres (absolute)/second is a value which represents a suitablelimit regarding both the hardness of the sand mold and the choice of thepressure medium.

The pressure surge wave should be applied to the surface of the sand asa mass flow through change of the amount of air per unit time. Thepressure surge may be produced by compressing a gas or by an exothermicreaction of an explosive gas mixture. The pressure surge wave should bereduced to a low final pressure, that is, finally reduced to normalpressure.

A pressure maximum of 4 to 5.5 atmospheres (absolute) in a combustionchamber represents an optimization between the desired strengthproperties of the sand molds and the cost of sealing the combustionchamber. .[.The maximum.]. .Iadd.An elevated .Iaddend.pressure .Iadd.ofat least 2 atmospheres (absolute) .Iaddend.should be maintained for atleast 0.01 second and may be .[.mainted.]. .Iadd.maintained .Iaddend.forabout 0.03-0.05 seconds.

With regard to the reduction of pressure, two oppositely directedphenomena must be weighed one against the other. On one hand, thepressure drop should not be too rapid, because cracks could form in themolds; whereas, on the other hand, too much time should not be taken forthe pressure to drop, otherwise the cycle times for producing the moldswould become too long. An upper limit would be in the range of about 2atmospheres (absolute)/second, up to which limit, crack-free molds witha clean surface can be produced. The maximum pressure is reduced in atime ranging from 0.2 sec. to about 1.5 sec.

In the afore-described process use is made of four parameters to definea pressure surge. These parameters are the increasing pressure gradient,the maximum pressure, the duration of maximum pressure, and thedecreasing pressure gradient. These parameters may be variedindependently.

The FIGURE shows four pressure surges, all of which are characterized byincreasing pressure gradients, maximum pressures, duration of maximumpressure, and decreasing pressure gradients in the aforementioned rangesin accordance with the present invention. Each of the surges starts atnormal, i.e. atmospheric pressure and ends at normal, i.e. atmosphericpressure.

Additional influencing factors were investigated to determine theabove-mentioned optimum values. Thus, a boundary surface concentrationof the raw material particles of 10⁻⁹ to 5·10⁻⁹ mole/centimeter², andespecially, moreover, a concentration of 1.9 to 2.5·10⁻⁹mole/centimeter², has been found to be advantageous.

In addition, with regard to making a choice of raw material particles, adetermination was made relating to the velocity of propagation ofelastic longitudinal waves in such particles, wherein a value of 4 to 7kilometers/second, and preferably, a velocity of 5.8 to 6.0kilometers/second, was found to be especially favorable.

For the tests, use was made of raw material particles which had a majorconstituent which was silicon dioxide. These particles had acompactability which lay between 27 and 60%, and preferably, 32 to 45%.

Compactibility relates to the decrease in volume of the casting mixtureafter the pressure surge is applied thereto.

Advantageously, the process described finds an application in theprecise molding of a casting mold by using a pattern wherein the derivedoptimum values are so adjusted and combined that a pressure of 80 to 180Newtons/centimeter² is produced on the surface of the pattern.

The limiting values thus determined for the pressure rise, for themaximum value and for the pressure reduction, yield data for the optimaloperation of equipment for compacting sand.

I claim:
 1. A method for manufacturing molds from casting sand or othermixture comprising raw material particles, binder and water, by applyinga pressure surge wave to said casting sand or other mixture in a closedchamber, said pressure surge wave being formed by a process comprisingthe steps of:(a) starting from atmospheric pressure in said chamber,increasing said pressure with a pressure gradient of at least about 50atmospheres/sec to a .[.maximum.]. pressure of at least about 2atmospheres; (b) maintaining said .[.maximum.]. pressure .[.of atleast.]. .Iadd.in said chamber equal to or above .Iaddend.about 2atmospheres for at least about 0.01 seconds; (c) reducing said pressure.[.from said maximum pressure.]. to atmospheric pressure within .[.atleast.]. about 0.2 seconds .Iadd.or more .Iaddend.from the initiation ofthe pressure increase.
 2. A process according to claim 1, characterizedin that the increasing pressure gradient has a maximum value of about600 atmospheres (absolute)/sec.
 3. A process according to claim 1,characterized in that the maximum value of the pressure p lies betweenabout 4 and about 5.5 atmospheres (absolute).
 4. A process according toclaim 1, characterized in that the maximum gas pressure is maintainedfor 0.03 to 0.05 seconds.
 5. A process according to claim 1,characterized in that the gas pressure is reduced within about 1.5seconds.
 6. A process according to claim 1, characterized in that thereduction in pressure is carried out using a decreasing pressuregradient up to about 2.0 atmospheres (absolute)/second.
 7. A processaccording to claim 1, characterized in that use is made of raw materialparticles having a propagation velocity for elastic longitudinal wavesof 4 to 7 kilometers/second, and preferably of 5.8 to 6.0kilometers/second.
 8. A process according to claim 1, characterized inthat use is made of raw material particles whose boundary surfaceconcentration is 10⁻⁹ to 5·10⁻⁹ mole/centimeter², and preferably1.9·10⁻⁹ to 2.5·10⁻⁹ mole/centimeter².
 9. A process according to claim1, characterized in that the raw material particles comprise mainlysilicon dioxide.
 10. A process according to claim 1, characterized inthat the pressure rise is effected by compressing a gas.
 11. A processaccording to claim 1, characterized in that the pressure rise isproduced by the exothermic reaction of an explosive gas mixture.
 12. Aprocess according to claim 1, characterized in that use is made of amixture with a volume capable of being compacted by 27 to 60% uponapplication of said pressure surge wave, and preferably by 32 to 45%.13. The use of the process according to claim 1, characterized in that amaximum pressure of 80 to 180 Newtons/centimeter², is produced on thesurface of a pattern. .Iadd.
 14. A method for manufacturing molds fromcasting sand or other mixture comprising raw material particles, binderand water, by applying a pressure surge wave to said casting sand orother mixture in a closed chamber, said pressure surge wave being formedby a process comprising the steps of:(a) starting from atmosphericpressure in said chamber, increasing said pressure with a pressuregradient of at least about 50 atmospheres/sec to a maximum pressure ofat least about 2 atmospheres; (b) maintaining said maximum pressure ofat least about 2 atmospheres for at least about 0.01 seconds; (c)reducing said pressure from said maximum pressure to atmosphericpressure within about 0.2 seconds or more from the initiation of thepressure increase. .Iaddend. .Iadd.15. A process according to claim 14,characterized in that the increasing pressure gradient has a maximumvalue of about 600 atmospheres (absolute)/sec. .Iaddend. .Iadd.16. Aprocess according to claim 14, characterized in that the maximum valueof the pressure lies between about 4 and about 5.5 atmospheres(absolute). .Iaddend. .Iadd.17. A process according to claim 14,characterized in that the maximum gas pressure is maintained for 0.03 to0.05 seconds. .Iaddend. .Iadd.18. A process according to claim 14,characterized in that the gas pressure is reduced within about 1.5seconds. .Iaddend. .Iadd.19. A process according to claim 14,characterized in that the reduction in pressure is carried out using adecreasing pressure gradient up to about 2.0 atmospheres(absolute)/second. .Iaddend. .Iadd.20. A process according to claim 14,characterized in that use is made of raw material particles having apropagation velocity for elastic longitudinal waves of 4 to 7kilometers/second, and preferably of 5.8 to 6.0 kilometers/second..Iaddend. .Iadd.21. A process according to claim 14, characterized inthat use is made of raw material particles whose boundary surfaceconcentration is 10⁻⁹ to 5×10⁻⁹ mole/centimeter², and preferably1.9×10⁻⁹ to 2.5×10⁻⁹ mole/centimeter². .Iaddend. .Iadd.22. A processaccording to claim 14, characterized in that the raw material particlescomprise mainly silicon dioxide. .Iaddend. .Iadd.23. A process accordingto claim 14, characterized in that the pressure rise is effected bycompressing a gas. .Iaddend. .Iadd.24. A process according to claim 14,characterized in that the pressure rise is produced by the exothermicreaction of an explosive gas mixture. .Iaddend. .Iadd.25. A processaccording to claim 14, characterized in that use is made of a mixturewith a volume capable of being compacted by 27 to 60% upon applicationof said pressure surge wave, and preferably by 32 to 45%. .Iaddend..Iadd.26. The use of the process according to claim 14, characterized inthat a maximum pressure of 80 to 180 Newtons/centimeter², is produced onthe surface of a pattern. .Iaddend.